In recent years, high-efficiency transmission using digitization has been employed in radio communications. Usually, a radio apparatus that performs such radio communications includes a power amplifier. The radio apparatus inputs a transmission signal to the power amplifier and then emits the transmission signal, whose power is amplified and which is output from the power amplifier, to the atmosphere via an antenna. Hereinafter, the power of the signal that is input to the power amplifier can be referred to as an “input power” and the power of the signal that is output from the power amplifier can be referred to as an “output power”.
Such a power amplifier has a characteristic that, when the input power is larger than a certain value, the relation between the input power and the output power is not liner. This characteristic will be described here using FIG. 9. FIG. 9 is a graph of an example of I/O (input/output) characteristics of the power amplifier. The horizontal axis in FIG. 9 represents the power of the signal that is input to the power amplifier and the vertical axis in FIG. 9 represents the power of the signal that is output from the power amplifier.
In the example illustrated in FIG. 9, when the input power is smaller than a certain value “PX”, the relation between the input power and the output power is linear. In contrast, when the input power is larger than the certain value “PX”, the relation between the input power and the output power is not linear. Specifically, when the input power is larger than the certain value “PX”, the output power is saturated. As described above, the I/O characteristics of the power amplifier can be divided into a “liner area” in which the relation between the input power and the output power is linear and a “non-linear area” in which the relation between the input power and the output power is not linear.
The signal that is output from the power amplifier having the above-described non-linear area contains a non-linear distortion, which leads to a problem that the communication quality deteriorates. This problem will be described here using FIG. 10. FIG. 10 is a graph of an example of frequency spectrums. The horizontal axis in FIG. 10 represents the frequency and the vertical axis in FIG. 10 represents the power. The solid line L11 in FIG. 10 represents the frequency spectrum of the signal on which power amplification is performed in the non-linear area and the dotted line L12 in FIG. 10 represents the frequency spectrum of the signal on which power amplification is performed in the linear area.
As illustrated in FIG. 10, sidelobe increases in the power of the signal on which power amplification is performed in the non-linear area compared with the power of the signal on which power amplification is performed in the liner area, and thus a power leakage to adjacent channels occurs. This is because a signal on which power amplification is performed in a non-linear area contains more non-linear distortions compared with a signal on which power amplification is performed on a liner area. Such power leakage deteriorates the communication quality of adjacent channels.
Some recent radio apparatuses include a distortion corrector that corrects a non-linear distortion contained in a transmission signal in order to prevent deterioration of the communication quality. Specifically, the distortion corrector performs a distortion correction process on an input signal, which is input to a power amplifier, using a distortion correction coefficient that is stored in a predetermined storage unit. The distortion corrector calculates an error signal between the input signal, which is input to the power amplifier, and a feedback signal, which is fed back from the power amplifier, and then multiplies the calculated error signal by a step-size parameter. The distortion corrector obtains an update value of the distortion correction coefficient by adding the multiplication result and the distortion correction coefficient, which is stored in the predetermined storage unit. The distortion corrector then updates the distortion correction coefficient, which is stored in the predetermined storage unit, to the update value.
The step-size parameter is a value for gradually updating the distortion correction coefficient and represents an updating rate of the distortion correction coefficient. In other words, the distortion corrector gradually updates the distortion correction coefficient, which is stored in the distortion correction coefficient storage unit, by multiplying the error signal between the input signal and the feedback signal by the step-size parameter.
When the step-size parameter is a large value, the variation amount of the distortion correction coefficient is large and thus the distortion correction coefficient may possibly not converge. Particularly, when the error signal is a large value, a step-size parameter that is a large value is multiplied by the error signal that is the large value. Accordingly, the variation amount of the distortion correction coefficient is large and thus the distortion correction coefficient may possibly not converge.
when the step-size parameter is a small value, the rate at which the distortion correction coefficient converges is slow. When the error signal is a small value, the step-size parameter that is a small value is multiplied by the error signal that is a small value. Accordingly, the error may be eliminated and thus the distortion correction coefficient may not be updated.
In recent years, a technology for adaptively varying the step-size parameter for each of predetermined ranges of input power has been developed. This technology may allow adaptively adjusting the rate at which the distortion correction coefficient converges for each of the predetermined ranges. A technology has been also developed in which the distortion correction coefficient is updated by dividing the complex conjugate signal of the transmission signal or the feedback signal by the amplitude value of the transmission signal or the feedback signal. This technology may realize stable convergence characteristics not depending on the amplitude level of the input signal.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-270246    Patent Document 2: Japanese Laid-open Patent Publication No. 2005-102029
However, the above-described conventional technology has a problem that the circuit scale increases. Specifically, in the conventional technology in which the step-size parameter varies for each of the predetermined ranges of the input power, multiple step-size parameters are stored and thus this leads to a problem that the memory size increases. In addition, in the conventional technology in which the complex conjugate signal is divided by the amplitude value of the transmission signal or the feedback signal, an I (in-phase component) signal and a Q (quadrature component) signal of the transmission signal, and an I signal and a Q signal of the complex conjugate signal are divided. In other words, four dividers are used to use this technology. This leads to the problem that the circuit scale increases.